This invention relates to coating compositions and more specifically to corrosion-resisting coating compositions which can be applied by various methods directly onto surfaces and particularly metal surfaces and/or polymeric or plastic substrates without the need for an undercoat.
Metal substrates, and particularly metal exposed to extreme environmental conditions, require the protection of coatings capable of resisting corrosive environments, e.g., ships and military aircraft are exposed to seawater spray and other corrosive agents including not only saltwater spray but various acid forming gases, e.g. sulfur dioxide and the like. In addition to ships and aircraft, particularly military aircraft, various types of machinery and farm equipment used in an industrial environment where fossil fuels generate corrosive agents need protection against reagents such as surfur and other acid forming gases. In addition to corrosion, it is important that the coatings useful in these environments be resistant to various chemicals and have physical characteristics which enable them to be applied to the substrate without difficulty. These coatings should also exhibit good adhesion and have a high degree of flexibility.
Presently, coatings attempting to comply with one or more of the above mentioned requirements rely on the use of a plurality of films, i.e. an undercoat with a topcoat, comprising, for example, an epoxy undercoat and a polyurethane topcoat. The epoxy primers used in the military are specifically designed to adhere to metal surfaces. These primecoats, however, generally require a topcoat, since the primers do not have the required flexibility, particularly at low temperatures, and therefore results in extensive cracking and/or blistering in areas which are highly flexible. Moreover, the primer coats are not generally resistant to harsh weather conditions and are difficult to formulate in the multi-colors required for military aircraft. The epoxy coating compositions of this invention, however, are not only resistant to harsh weather conditions and the various chemicals including saltwater and acidic components, but have also the required degree of flexibility. In addition, it was necessary heretofore to provide a plurality of films of the coatings thereby forming a total dry film thickness ranging up to about 0.005 inches e.g. up to 10 mils or greater which adds considerable weight to the aircraft. Thus, not only does the multi-coat finishes utilized heretofore require a plurality of film thickness but are also very time consuming in applying since it is necessary to allow sufficient drying time between each application. Moreover, it is obvious that the removal of the two coat system can likewise be difficult and time consuming and requires the use of large amounts of organic solvents causing objectionable emissions.
In accordance with this invention, the corrosion resistant costings comprise an epoxy binder, i.e. an epoxy resin such as the bisphenol-A epichlorohydrin condensation products which have molecular weights ranging up to 300,000. More specificallly, the coating compositions of this invention comprises approximately 100 parts by weight of an epoxy resin in combination with about 0 to 1000 parts by weight of at least one organic paint solvent for said resin and from about 0 to 140 parts by weight of a TiO.sub.2 pigment, i.e. titanium dioxide pigment in the form of vesiculated beads or combinations of TiO.sub.2 beads and pigment and from about 0.01 to 300 parts by weight of a combination of corrosion-inhibiting pigments consisting essentially of about 10 to 120 parts by weight of at least one zinc phosphate, 40 to 260 parts by weight of zinc molybdate, and 1 to 30 parts by weight of at least one zinc salt of a benzoic acid, e.g. substituted zinc benzoate. For purposes of this invention, all three of the zinc salts, as disclosed herein, are essential in their relative proportions to provide the corrosion resistance required for coatings. Other pigments, and particularly, spherical TiO.sub.2 particles and the vesiculated beads e.g. TiO.sub.2 beads may be used in combination with the three zinc salts as disclosed herein.
The coating composition of this invention maybe applied as a single coat directly onto a hard surface such as metal, plastic or polymeric surfaces and do not require an undercoat or a top coat to provide a high gloss, corrosion resistant film. It is generally known that low gloss coatings are appropriate for camouflage purposes particularly on most of the outer exposed surfaces of military aircraft and the like. On the other hand, low gloss coatings are not appropriate for the internal or unexposed surfaces such as the areas around engine inlets, ducts, landing gears, etc. Moreover, aircraft other than the military aircraft, require high gloss and high visibility coatings. It was therefore believed that in order to obtain a coating exhibiting outstanding corrosion resistance, the amount of pigment, i.e. (PVC) pigment volume concentration had to be realtively high which would therefore result in a low gloss finish. It was believed that it was not possible to obtain a final coat which had a high gloss and good corrosion resistance at a high pigment volume concentration.